Up to now, an apparatus with a speaker has been known, the speaker including a generating circuit for generating a sound signal and making a sound by amplifying a sound signal generated in the generating circuit by an amplifier. It has been also known that the apparatus with a speaker is mounted with a DC/DC converter for converting a DC voltage inputted from main power supply into two different kinds of DC voltage to output the two different kinds of converted DC voltage to the generating circuit as a power supply voltage.
On the other hand, as for the DC/DC converter, JP-A-2004-503197 discloses a multiple output DC/DC up-converter for independently controlling each output voltage by any of pulse width modulation (PWM) and pulse frequency modulation (PFM) for the purpose of highly efficient operation.
In the case of using the output voltage of the above-mentioned multiple output DC/DC up-converter disclosed in JP-A-2004-503197 as a power supply voltage for the above-mentioned generating circuit of the speaker, however, there is a problem as follows. That is to say, a difference in the frequency component between the two different types of output voltage overlaps with a sound signal generated in the generating circuit resulting in beats, which are outputted from the speaker as an unpleasant sound especially when the frequency difference is within an audible range, since each output voltage outputted from the multiple output DC/DC up-converter is independently controlled by any one of pulse width modulation (PWM) and pulse frequency modulation (PFM).